The present invention relates to a disposable absorbent article using a non-woven fabric having excellent absorptivity and processability as the surface material. The present invention particularly relates to an absorbent article such as a sanitary napkin, a paper diaper or a sheet for makeup.
A conventional absorbent article such as a sanitary napkin or a paper diaper basically comprises an absorbent layer made of cottony pulp or absorbent paper, a leakproof layer disposed on the lower and lateral sides of the absorbent layer, and a non-woven fabric provided on the surface of the absorbent layer.
The absorptivity of the absorbent article has been greatly improved in recent years since new materials such as a superabsorbent polymer and a dry process non-woven fabric have been introduced. Particularly as to the non-woven fabric, synthetic fibers have begun to be predominantly used in place of a regenerated fiber of cellulose which has heretofore been widely used. The synthetic fiber is very effective in suppressing the tackiness of the surface of the non-woven fabric.
An absorbent article comprising a combination of materials each having excellent absorptivity cannot be said to sufficiently exhibit the innate performance in actual use. This is apparent from the fact that the consumer's most serious dissatisfaction with an absorbent article such as a sanitary napkin or a paper diaper remains "leakage" from the crotch of a wearer.
The principal causes of leakage are separation of each of the constituent materials and great slippage or wrinkling of the absorbent article due to irregular stresses which are applied to the absorbent article when the crotch of the wearer moves. Among others, particularly the non-woven fabric receives great stress since it is directly in contact with the skin of the wearer so that it is likely to be separated from a waterproof paper layer of an absorbent layer. Thus, integration of the layers by any method is earnestly desired.
As a means of integration of the non-woven fabric with a waterproof paper layer or an absorbent layer, it is conceivable to bond them with a pressure-sensitive adhesive or a hot-melt adhesive. If such a means is employed, the production steps are very complicated, inevitably largely increasing the production cost. In contrast, if a so-called heat bonding method, in which a non-woven fabric is melted and bonded to an object by simple hot-pressing, is employed, the process becomes less complicated, and high-speed production becomes possible with a slight increase in production cost.
In view of the above, a non-woven fabric having good thermal processability is necessary for improving the leakproofness of an absorbent article under conditions of movement of the wearer while not increasing the cost. For high-speed production of such an absorbent article, other excellent processabilities are required of the non-woven fabric in addition to the thermal processability.
The synthetic resin non-woven fabric which has been predominantly used has very insufficient processabilities though it has excellent absorptivity. The problems can be broadly summarized as follows.
A first problem is that thermal processing of the non-woven fabric is difficult. When the non-woven fabric is made of a fiber incapable of being heat-melted, such as an acrylic fiber, the fabric cannot be bonded to an object at all because the non-woven fabric cannot be melted. When a non-woven fabric made of a polyester fiber, a nylon fiber, or the like having a high melting point can be melted by heat, control of the temperature is difficult due to the very high processing temperature, and damage to other materials may be great when the molten non-woven fabric comes in contact with them. Thus, even when the non-woven fabric is melted in processing, the following difficulties are experienced.
The molten fiber sticks to a heat sealer, with the result that the heat-bonded portion of the non-woven fabric is broken and that the function of the sealer is deteriorated because of the resin sticking to the sealer. Even when a low-melting fiber is partially blended in a non-woven fabric in order to lower the processing temperature, the situation is the same as if all of the low-melting fibers are molten. In most cases, the low-melting fiber used herein is one comprising components all having the same melting temperature, such as a polypropylene fiber, or one comprising components having a small difference in melting temperature from one another, such as a polyethylene/polypropylene conjugate fiber. When processing is conducted at a high temperature enough to provide sufficient heat-bonding strength, all components are molten at the same time, and the melted component instantly migrates to the sealer, leading to a reduction in the function of the sealer and to breakage of the heat-bonded portion. As the blending rate of the low-melting fiber is increased, this problem is more serious. When the blending rate of the low-melting fiber is low, migration of the molten component to the sealer can be somewhat prevented by bonding and intertwining of the low-melting fiber with the high-melting fiber in a temperature range wherein all of the low-melting fibers are molten but the high-melting fibers keep the fibrous form. However, the effect of thermal adhesion is hardly exhibited because of the low blending rate of the molten component, and migration of the molten component to the sealer remains yet to be eliminated. Thus, the breakage of the heat-bonded portion of the non-woven fabric and damages caused by the deposited matter accumulated in the sealer by running the sealer particularly for a long period of time cannot be essentially obviated.
There has been an attempt to improve the above-mentioned problems by using as the low-melting fiber a polyester/polyester conjugate fiber having a large difference in the melting temperature between the two components of the fibers. In this case, a conventional low-melting polyester component is so amorphous that it does not have a melting temperature (but has a softening point) in a strict sense. Thus, adhesion to another object is low and hence an insufficient sealing strength can be obtained even at a temperature above the softening point.
As described above, thermal processing of a non-woven fabric based on a synthetic fiber can merely be conducted in a very restricted range. Particularly, it is impossible to conduct thermal processing which will exert a consistent effect on a non-woven fabric having a very high blending rate of a low-melting fiber used as the binder, such as a dry process heat-bonded non-woven fabric which is widely used in absorbent articles.
Thus, in the most widely employed method of providing thermal processability to a synthetic fiber-based non-woven fabric, a polyolefin fiber or its conjugate fiber having a relatively high bonding strength when thermally processed is blended within a range wherein the fiber does not fuse as much to a sealer. However, this method can not essentially solve the above-mentioned problem, and involves the following serious problem.
Specifically, a second problem is that when a polyolefin fiber is used, the cuttability of a non-woven fabric is lessened. In production of a disposable absorbent article such as a sanitary napkin or a paper diaper, the step of cutting a product or a raw material does not fail to be included in the process. Generally speaking, the cuttability of a synthetic fiber is poor when compared with a regenerated cellulose fiber such as rayon. A high-speed productivity is required with the above-mentioned absorbent article for lowering the cost. Thus, cutting must be done in a very short time and the cutting system is limited. In a cutting system using a metal blade, which is usually employed, the durability of the blade is greatly affected by the properties of the materials such as a non-woven fabric. On the non-woven fabric side, the fabric should not undergo damages to a portion other than the cut portion It is believed that the cuttability has something to do with the viscoelasticity of the fibers. The cuttability of a polyolefin fiber having a high viscoelasticity is particularly poor, while those of a polyester fiber or an acrylic fiber having higher brittleness are relatively good. Accordingly, a dry process heat-bonded non-woven fabric based on a polyolefin fiber and a non-woven fabric containing a polyolefin fiber blended for providing thermal processability is difficult to process. Thus, difficulty is encountered in high-speed production of an absorbent article.
As described above, a conventional non-woven fabric based on a synthetic fiber does not simultaneously satisfy sufficient thermal processability and cuttability under conditions of high-speed production. Thus, an absorbent article which hardly slips or wrinkles and has a high leakproofness has not been obtained.